Waterflooding of portions of an underground reservoir to assist in producing heavy oil from underground hydrocarbon-containing reservoirs has been employed in the past.
Specifically, in a prior art application of the method of water flooding for assisting in producing oil from a formation, using a vertical production well to produce oil from an underground oil-containing formation, water is injected via vertical injection wells surrounding the single oil production well, in an attempt to maintain pressure in the reservoir (also known as voidage replacement) and/or sweep or displace the oil from the reservoir and push it towards the vertical oil production well, where it can then be produced to surface.
Waterflooding using horizontal wells as opposed to vertical wells was introduced by Taber in 1992 as a method for improving the performance of conventional waterfloods. The rationale for this geometry is that water can theoretically be injected at much higher rates and lower pressures in horizontal wells than in vertical wells, allowing oil to be recovered quicker. In one embodiment of the prior-art horizontal waterflooding process, a central horizontal water injection well is provided, adjacent to which are provided two parallel horizontal producing wells. The basic technique concept employed is that a large amount of water can be injected into the horizontal injector well at pressures that are below the fracture-parting pressure, displacing the oil laterally outwardly from the horizontal water injector well, to allow such migrated oil to then be recovered in each of the parallel horizontal producing wells.
Moreover, waterflooding is ineffective in bitumen containing formations, as bitumen does not flow unless heated, and in particular unless heated to temperatures much higher than original formation temperature.
As an alternative oil recovery method, steam-based oil recovery methods are commonly employed to recover heavy oil and particularly bitumen. For example, steam-assisted-gravity-drainage (SAGD) and cyclic steam stimulation (CSS) are used for the recovery of heavy oil or bitumen.
In a SAGD (Steam Assisted Gravity Drainage) method of oil recovery, a horizontal steam injector well is drilled relatively high in a hydrocarbon-containing formation, and a parallel horizontal production well is drilled low in the formation, having a horizontal portion typically situated directly below the horizontal portion of the injector well. Steam is injected into the formation via the horizontal portion of the injector well, and oil within the formation which becomes heated thereafter becomes mobile and by force of gravity drains downwardly in the formation, where it is collected by the horizontal production well and produced to surface.
In a cyclic steam stimulation (CSS) method, one or more wells are drilled into a development region of a hydrocarbon-containing reservoir. Steam is initially injected into the well(s) for a period of time to heat bitumen and heavy oil in a region of the formation surrounding the well(s). After a time injection of steam is stopped, and oil which has been heated and rendered mobile is allowed to drain into the well, and is produced to surface. The cycle is repeated numerous times.
Due to high levels of oil recovery (substantially greater than 30% of OOIP), SAGD and CSS oil recovery methods are often a superior means of producing oil from an underground reservoir, particularly where heavy oil and in particular bitumen deposits are encountered.
Disadvantageously, however, oil recovery percentages using only SAGD or CSS recovery methods are typically in only in the range of about 50% recovery (depending on factors including reservoir quality and thermal properties, and the like). Moreover, and also disadvantageously, the Steam/Oil ratio (SOR) with respect to SAGD and CSS methods is often very high, meaning that considerable expense and effort need be undertaken when using SAGD or CSS recovery methods to heat significant quantities of water to produce large volumes of steam in order to obtain the higher rates and levels of oil recovery. In addition, in numerous locations where heavy oil reservoirs may exist, sources of water may be rare or legislatively restricted due to environmental concerns regarding consumption of water to produce large quantities of steam.
Thus new methods of oil recovery are needed to reduce the SOR ratio, and reduce volumes of water needed in SAGD and CSS recovery methods.
Specifically, a real need exists for a method of oil recovery which achieves as high (or higher) a percentage of recovery of original oil in place (OOIP) as current SAGD or CSS methods, but which has a lower steam/recovered oil ratio and thus a lower operating cost to achieve such percentage recovery levels and/or rates of recovery.